The ERR_HTTP_HEADERS_SENT
error occurs in Node.js and Express applications when an attempt is made to modify the HTTP headers of a response after they have already been sent to the client. This can happen if you’re trying to send a response to the client but the response has already been transmitted, or if a response is being sent multiple times.
Reasons Behind the Error
- Multiple Responses: Sending more than one response to a client request.
- Async Operations: Attempting to set headers after an asynchronous operation has completed and a previous response has already been sent.
Steps to Fix the Error
- Be sure to only send one response per request.
- Use return statements to prevent further code execution after sending a response.
- Check your route handlers for any unintended multiple response calls.
- Ensure you handle errors properly so that they don’t lead to additional responses being sent.
Complete Code Example
const express = require('express');
const app = express();
app.get('/', (req, res) => {
// Imagine some logic here that may result in sending a response
res.send('First response');
// More logic here that attempts to send another response by mistake
// This will cause ERR_HTTP_HEADERS_SENT
// res.send('Second response'); // Comment this out to prevent the error
});
app.listen(3000, () => {
console.log('Server is running on port 3000');
});
Additional Solutions
If your code involves asynchronous operations (like database queries), be sure to handle the response only once. For example:
app.get('/user', (req, res) => {
User.findById(req.params.id, (err, user) => {
if (err) {
// Handle the error and send a response
return res.status(500).send(err);
}
if (user) {
// Send user data as a response
res.send(user);
} else {
// Handle case when user isn't found
res.status(404).send('User not found');
}
// No further response should be sent here!
});
});
Every response method (like res.send
, res.json
, res.end
) should be called once per request. Using return
with these methods can help prevent further execution which might cause additional response attempts.